Buy any three titles from the list contained in the link and you get 800 Microsoft points back.
The bare minimum to spend would be US$7 consisting of both Mass Effect downloads, plus the Dragon Age: Origins DLC, for 1360 points. You then get 800 back, effectively making the cost 560 points.
Have you tried this? I'm suspicious that this won't work for Australian accounts. I couldn't find any details of this on the official EA Australia website.